In a fluid handling device, such as a pump or a valve, where a moving stem or shaft extends through a wall of the device, a seal is required at that point to prevent the fluid from leaking from the device. Leakage from such fluid handling devices is undesirable for obvious health, air quality, and safety reasons. For example, a leak of a toxic or flammable fluid could pose a direct threat to human life. Today's heightened environmental consciousness is another influential driving force behind minimizing leaks of toxic or other potentially harmful liquids or gasses onto the ground or into the atmosphere.
Accordingly, such fluid handling devices are often sealed by placing a formed packing material around the shaft, and containing the packing material in the compressed state in a stuffing box. Ideally, the packing material selected should be resilient such that it deforms under compression to conform to the interior of the stuffing box and forms a tight interference seal against the shaft. The packing material should also present a low friction surface to the moving shaft and be stable under the environmental conditions to which it may be exposed. It is also desirable that the packing material act to keep the shaft clean and clear of debris by wiping the surfaces of the shaft as the shaft is passed through the stuffing box. Preferably, the packing is a self energized seal, i.e., that it seals by application of pressure on the seal. It is also desirable that the packing material itself be resistant to fire since many applications are for petrochemical service where fire may be a concern.
Flexible graphite is known in the art and has long been employed as a packing material to form seals for the stuffing box assembly of pumps, valves and like fluid handling devices. Flexible graphite refers to graphite which has been exfoliated and recompressed to a coherent body. The advantages of using graphite as a packing material lies in its excellent thermal stability and chemical resistance. Graphite is also a low friction composition that has commonly been used as a lubricant in certain applications. However, as practiced in the art, flexible graphite has not always proven to be an adequate sealing material due to the lack of resiliency inherent in the particular form used.
One such form of flexible graphite is that of a preshaped ring made by compressing in a closed die a ribbon or tape of graphite that has been wrapped circumferentially in several layers around a shaft. This spiral wrapped form of flexible graphite comprises an anisotropic structure having its bonding planes oriented parallel to the shaft axis. Such flat rings may also be made by laminating exfoliated graphite particles or sheets in a flat sheet and cutting flat gaskets from such a sheet.
The rings are used by stacking several rings over the stem of the fluid handling device such that the rings occupy the annular space between the shaft and the stuffing box housing. A metal collar is then inserted over the stem and is tightened to compress the graphite rings so that the rings deform laterally. An interference fit is formed against the shaft and the interior wall by applying a compressive force to the top of the ring stack.
A shortcoming of this form of flexible graphite lies in its limited resiliency when subjected to an axial compression force. The ends of the ring are flat and they spread laterally only a small amount in response to compression. This lateral spreading is controlled by Poisson's ratio for the material. High compressive forces are required to maintain a good seal. In addition, only the rings near where the force is applied seal due to poor transfer of load between the rings. In one type of valve, a compressive force of about 400 kg/cm.sup.2 (5600 psi) may be required to get an interference fit. Even so, traces of leakage may be detected immediately or after limited use.
Flexible graphite seals made from exfoliated graphite have also included braided graphite yarns. Such seals are typically made by wrapping the braided graphite around a shaft. Individual rings may be formed by cutting a helix of braided graphite.
A somewhat self energizing seal is made with a stack of rings, each of which has a wedge shaped transverse cross section. Alternating rings have greater thickness at the inside diameter and outside diameter, respectively. Longitudinal compression on a stack of such rings in a seal tends to wedge alternating rings inwardly and outwardly for sealing against the shaft and stuffing box, respectively.
A self energizing seal which is better than a flat ring has been made of materials such as polytetrafluoroethylene (Teflon). In cross section each side of the ring has a chevron shape. The angles of the chevron are different on the opposite end faces of the ring. When the rings are compressed, the concave side of the chevron is spread laterally by the convex side and the edges of the chevron tightly engage the shaft and stuffing box, forming a tight seal. Teflon forms a good seal, but has temperature limitations and cannot be used above about 260.degree. C.
It would be desirable to form graphite chevron seals, but they have never been made satisfactorily. When such seal rings are formed from circumferentially wound graphite, the planes of weakness extend in the direction of the shaft axis and the rings break apart during the forming operation or when stressed during use.
It is, therefore, highly desirable to provide a flexible graphite packing ring that is sufficiently resilient to allow the degree of deformation necessary to provide a tight interference fit under compression and be strong enough to resist compressive forces. It is also desirable that the flexible graphite packing material be both simple to install and operate.